'IRAQ AND THE PERSIAN GULF' [‎27v] (59/862)

26 GEOLOGY AND DESCRIPTION OF THE LAND
in south-east Turkey and of the three principal tributaries which
reach it in Iraq: the Great Zab, the Little Zab, and the Diyala.
Thus at Mosul only the Turkish tributaries affect its volume; at the
Fat-ha gorge and at Baghdad all but the Diyala: and at Kut al Imara
all of them, though some water has already been lost by irrigation,
and some has been added by subsoil drainage from marshland. As
with the Euphrates the river is lowest from mid-September onwards,
but late autumn rainfall begins to affect it often before the beginning
of November, and from then through the winter heavy rainfall may
occur at any time and cause sudden rises and fluctuations. If only
one tributary is in spate at a given time, the increased volume of the
Tigris may be more noticeable in the increased velocity than in the
rise; but when all the tributaries bring down storm-water, there may
be almost as much danger to the embankments then as later. These
winter flood-levels are, however, usually of short duration. When
the main discharge comes from the melting snow, the rise is steadier
but reaches a greater maximum in March or April, often a month or
more before high flood of the Euphrates. At Baghdad the normal
difference between high- and low-river levels is about 18 feet, but as
much as 9-19 feet caused by rainfall has been recorded in 24 hours.
At the same place the mean annual discharge is 43,800 cubic feet
(1,240 cu. m.) per second, a much higher figure than that of the
Euphrates; it varies between a mean monthly low-water discharge of
11,900 cubic feet (337 cu. m.) per second and a flood discharge of
106,650 cubic feet (3,020 cu. m.). The difference between the two
rivers is very noticeable in the flood season, when the Tigris between
narrower banks may be discharging twice as much water per
second as the Euphrates.
(c) The Euphrates in Iraq
The Euphrates enters Iraq (fig. 5) below Abu Kemal (34 0 25' N.,
40 57 E.). Above this point the steep-sided valley which it has
carved in the nearby horizontal beds of the stony platform of the
Jazira is nearly 20 miles wide, but the river has cut its meandering
course in the valley bottom, so that its banks are steep and there is
little space available for cultivation. Currents here are swift and the
water deep, especially in the flood season, and navigation is difficult.
Ahu Kemal to Ana (direct distance 59 miles; by river 100 miles)
Throughout this section the Euphrates is less than 600 feet above
sea-level; its average depth is about 12 feet in the low-water season,